1. Field of the Invention
The present invention relates to a digital feedback linearizing apparatus and method to linearize a power amplifier (PA) to improve the linearity of a mobile communication base station PA, and more particularly, to a digital feedback linearizing apparatus and method to linearize a PA using a digital signal process (DSP) and a feedback technology, in which a predistorted signal required for linearization is generated by adding an input signal input through a predetermined path to an inverse distortion component corresponding to a distortion component of a PA and in which a linearly amplified output signal is obtained by passing the predistorted signal through the PA, so that linearity of the PA is effectively improved.
Also, the present invention relates to a digital feedback linearizing apparatus and method, which generates the optimal predistorted signal for an input signal by obtaining an inverse distortion component corresponding to the amplitude of the input signal from a previously formed and stored lookup table.
2. Description of the Related Art
As is well known to those skilled in the art, mobile base station power amplifiers preferably output high power signals without distortion. In particular, since a wideband code division multiple access (WCDMA) technology adopts multi-carrier (MC) transmission, standards for linearity become increasingly important. To ensure superior linearity, various linearizing techniques and apparatuses adopting such techniques such as analog predistorters, analog feedback linearizers, feedforward linearizers, and digital predistorters have been used.
Among such linearizing apparatuses, analog predistorters and analog feedback linearizers have narrowband frequency characteristics and have trouble in linearizing input signals to target values. Feedforward linearizers have been most commonly used so far and have superior linearizing characteristics, but have problems such as low cost-competitiveness, high heat emission, and large size.
To solve the above problems, digital predistorters have been developed, which use digital signal processing, have a wide frequency band, show superior linearity improvement, have high cost- and size-competitiveness, and are very suitable for base station linearizers. However, if the digital predistorters themselves have many errors, error tolerance of PAs in which they are used is not large. As a result, it is difficult to cancel distortion of the PAs as desired by standards.
A conventional analog feedback predistorter used as a linearizer for a PA which is shown in FIG. 1, can efficiently extract predistorted signals, has very large error tolerance in cancellation of distorted signals. Referring to FIG. 1, the conventional analog feedback predistorter comprises a feeding block 2, a canceling block 4, a main amplifier block 6, and vector modulators (VM) 3 and 5. In the canceling block 4, an input signal u(t) of a main PA 10 is cancelled from an output signal y(t) of the main PA 10 and an inverse distortion component e(t) is extracted. In the feeding block 2, the inverse distortion component e(t) is added to an input signal x(t) through a feedback loop. In FIG. 1, the vector modulators 3 and 5 properly modulate the phases of respective input signals.
A frequency domain signal Y(f) of the output signal y(t) output from the conventional analog feedback predistorter of FIG. 1 is expressed as follows.
                    Y        =                              X                                                            (                                      1                    -                    Gu                                    )                                /                Gm                            -              Gy                                +                                                    (                                  1                  -                  Gu                                )                            ⁢                                                          ·              Xd                                      1              -              Gu              -                              Gy                ⁢                                                                  ·                Gm                                                                        (                  1.          ⁢          a                )                                          Y          ≈                                    X              Gy                        -                                                            (                                      1                    -                    Gu                                    )                                ·                Xd                                            Gy                ·                Gm                                                    ,                            (                  1.          ⁢          b                )            
where Gm, Gu, and Gy respectively denote a gain of each path shown in FIG. 1, X denotes an input signal of a frequency domain, and Xd denotes an intermodulation (IM) signal generated by the main PA 10.
The first term of Equation 1.a indicates an amplified main signal and the second term indicates a cancelled IM signal. To completely cancel the IM signal, the second term should be made to approximate 0. To this end, it is desirable to set Gu to 1. Assuming that Gu approximates 1, Equation (1.a) can be expressed as approximation Equation (1.b) that clearly shows a feedback operation. The total gain GPD of the analog feedback predistorter is determined by the gain of a feedback loop and is equal to −1/Gy. An IM component is divided by the gain Gy·Gm (≧1) of a closed loop due to a negative feedback operation. If the analog feedback predistorter of FIG. 1 is designed so that Gy·Gm is much greater than 1, cancellation of the distortion component is maximized and distortion cancellation error tolerance increases.
However, due to feedback characteristics, the conventional analog feedback distorter has a narrow operating band and operates unstably, resulting in a high possibility of oscillation.